In general, industrial robots use a speed reducer to convert high-speed low torque that is output from a drive motor into low-speed high torque for driving their joints. As the speed reducer used therefor, there has been known a strain wave gearing that utilizes differential motions of an ellipse and a perfect circle. In the strain wave gearing, a large number of teeth mesh with each, other at a time, and hence a high torque capacity can be obtained. Due to this advantage, the strain wave gearing is used in various industrial robots. However, the strain wave gearing has disadvantages of high cost and poor durability due to utilization of deformation.
Besides, as the speed reducer, an oscilating gearing that provides a high reduction gear ratio by using a rocking movement of a rocking gear therein is also generally used. In the oscilating gearing, a fixed, gear is provided coaxially with an input shaft, and the rocking gear having a different number of teeth is inclined by the input shaft and meshed with the fixed gear. The rocking movement is performed by rotation of the input shaft. At this time, the rocking gear revolves per rotation of the input shaft by an amount corresponding to a difference in number of teeth, and hence only a component of the revolution is transmitted to an output shaft. In this way, speed reduction is performed (PTL 1). Further, in another example, speed reduction is performed not by transmission of the component of the revolution bar by differential motions of a pair of the fixed gear and an output gear that is provided opposite thereto, and a pair of the output gear and the rocking gear that is meshed with the output gear (PTL 2).